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Sea State

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Category: General General
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Definition

The term Sea State describes the general condition of the surface of the open sea. This is comprised of two core measurements, the wave height which is dependent on the local surface wind strength and thus termed the ‘wind sea’ and the swell which is a slow and regular movement of the sea in rolling waves that do not break and are in the process of dissipating as they travel from their relatively distant origin.

Discussion

In the context of aviation, sea state may clearly be of significance to seaplanes and flying boats but for land based aircraft, the main relevance is to the controlled ditching of both fixed wing and rotary wing types. Where the latter are routinely operated over water, they are usually equipped with floatation devices which are designed, in up to a limiting sea state, to keep the aircraft upright to facilitate occupant evacuation.

Measurement

Both wave height and swell wavelength may be estimated by a trained observer but nowadays are more likely to be measured by instruments on buoys or by satellite remote sensing. Instrument measurements at buoys must be configured to average over a duration which is greater than the individual wave period or interval but less than the period over which the wind and swell conditions vary significantly. Automatic equipment is typically set to record measurements based on an interval of between one hundred and one thousand wave-periods.

Sea State Observation Scales

Scales for the range of both wave height and swell wavelength were developed and adopted by the Royal Navy in the 1920s and collectively became known as the Douglas Sea Scale. Subsequently, the World Meteorological Organisation (WMO) adopted only the “wind sea” table of wave heights from the Douglas Sea Scale as the WMO Sea State Code:

Code Height (m) Description
0 no wave Calm (Glassy)
1 0 - 0.10 Calm (Rippled)
2 0.10 - 0.50 Smooth (Wavelets)
3 0.50 - 1.25 Slight
4 1.25 - 2.50 Moderate
5 2.50 - 4.00 Rough
6 4.00 - 6.00 Very Rough
7 6.00 - 9.00 High
8 9.00 - 14.00 Very High
9 14.00+ Phenomenal

Uses and Limitations of Sea State Codes

The routine aviation use of Sea State Codes is largely limited to over sea helicopter operations. On the rare occasions when fixed wing aircraft, especially large ones, carry out a controlled ditching in the open sea or when rotary wing aircraft do the same, an awareness of the Code will at least provide the pilots some understanding of the prevailing surface conditions. Rotary wing pilots know that they are much more likely to have to ditch their aircraft than fixed wing pilots because their Quick Reference Handbook (QRH) will contain a considerable number of drills with a requirement to ‘land immediately’, even whilst over the sea, if certain critical malfunction warnings are annunciated. For this reason, commercially operated rotary wing aircraft are likely to be fitted with floatation equipment, although their inherent instability when floating on the sea surface, caused by their high centre of gravity, means that they are unlikely to remain upright in higher sea states. The prevailing sea state will be also be relevant to the process of occupant evacuation from a ditched helicopter (or one which has sustained a survivable uncontrolled ‘water impact’) before any capsize occurs.

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